This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. 1) One system is involved in sialyltransferases, or enzymes that add sialic acid onto acceptor carbohydrates. Sialyltransferases are key enzymes that catalyze the synthesis of sialic acid-containing oligosaccharides, polysaccharides, and glycoconjugates that play pivotal roles in many physiologically and pathologically critical processes in both prokaryotic and eukaryotic organisms. Despite their important biological functions, little is known about the molecular architecture and catalytic mechanism of these enzymes. Our long-term goals of this project are to elucidate the molecular structure and catalytic mechanism of STs and to develop ST inhibitors as potent therapeutics that can be used to treat cancers and pathogenic bacterial infections. 2) Another cellular pathway the lab is investigating is in sulfur assimilation and the sulfate activation pathway. The lab has determined a number of enzyme structures in this pathway from diverse organisms using data collected at SSRL. This proposal will focus on better understanding the structural requirements for regulation of the first two enzymes in the pathway as well as subsequent enzymes involved in the synthesis of sulfur containing biomolecules. 3) Finally, a new project the lab has initiated recently is investigating the structural features on plant and photosynthetic bacteria enzymes involved in chromophore synthesis. In particular, synthesis of phytochromes that regulate many cellular processes in response to light. PcyA, a representative ferredoxin-dependent bilin reductase is responsible for the formation of phycocyanobilin (PCB), the precursor of the chromophores of the photoreceptor phytochrome and a number of phycobiliproteins. This PcyA enzyme reduces biliverdin by four electrons, and proceeds through two radical intermediates, which have been confirmed by EPR spectroscopy. We have generated mutants that extend the life time of these radicals to many hours. Radical enzyme has been formed in the crystal and confirmed by EPR.